Lars Breuer oral presentation (PB2-Thu3-2-4)
Imaging with IR strong-field post-ionization
1 The Pennsylvania State University, 104 Chemistry Building, PA 16802 University Park, United States
2 Universitäät Duisburg-Essen - Fakultät für Physik, Lotharstr. 1-21, 47048 Duisburg, Germany
The role of imaging in the SIMS community has become more and more important. While the spatial and depth resolution increases, the amount of material available for analysis decreases. Due to this, the low ionization probabilities in a SIMS experiment ultimately limit the achievable resolution. One way to overcome that limitation is to increase the amount of detectable material available by post-ionizing the neutral counterparts of the sputtered material. This approach not only helps to increase signals, but also helps to reduce or overcome matrix effects on the ionization probability.
Lasers have been found to be a suitable tool for post-ionization. They are not only free from limitations by space-charge on the photon density, but photo-ionization is also a soft method to ionize sputtered neutral species without causing severe fragmentation of sputtered molecules or clusters. Our lab is using IR strong-field ionization (SFI) due to its universal ionization capabilities and reduces amount of photo-fragmentation compared to other methods of post-ionization. The drawback of this method is the need for a tightly focused laser beam to reach the power densities necessary (>1013 W/cm2) to perform SFI. This tight focusing causes under-sampling of the plume of sputtered particles, i.e. only a fraction of the available neutral material is probed by the laser beam. If the ion beam is scanned as in an imaging experiment, the tightly focused laser beam only ionizes effectively sputtered material in the center region of the image and can be seen as a bright line.
Here, we present a strategy to avoid that issue arising from the tight focusing of the laser beam. In a first approach, a series of images was collected with the laser beam set statically to different positions relative to the center of the sample. These images were then summed to produce a homogenous image of the sample. In a next step, the movement of the laser beam was automated and dynamically synchronized with the movement of the primary ion beam. This technique was used to image and depth profile different structures on organic layers.